The present invention relates to a heat exchanger adapted for a heat exchange between air and a heat carrying medium, for use in air-conditioning system, refrigerating system, dehumidifier and the like and, more particularly, to a cross-fin tube type heat exchanger having a multiplicity of fins provided with a large number of louvers formed therein.
A typical conventional cross-fin tube type heat exchanger has a multiplicity of fins made of aluminum sheets of a predetermined area and having a plurality of bores for receiving heat transfer tubes. These fins are disposed in parallel with one another such that the bores of these fins are axially aligned with corresponding bores of the other fins. A plurality of heat transfer tubes are inserted into respective bores of the fins, and are closely fitted and fixed to the latter by means of tube expansion or the like measure. The ends of the tubes projecting out of the outermost fins are connected by means of U-bent tubes so as to form a suitable number of continuous winding heat transfer tube passages.
A heat exchanging fluid or medium such as cold water, hot water, refrigerant or the like is made to flow in thus formed heat transfer tubes, while another heat exchanging fluid or medium, typically air, is made to flow through the gaps between adjacent fins at a suitable flow velocity, whereby heat is exchanged between two fluids or mediums across the tube wall and fins.
The cross-fin tube type heat exchanger having above-stated construction in one hand enjoys advantages of comparatively large area of heat transfer and reduced size, but, on the other hand, possesses the following problems.
In the heat exchange between two mediums flowing inside and outside of the tubes, boundary layers of the medium flowing outside the tubes, e.g. air, are formed along the surfaces of the fins and seriously deteriorates the heat transfer. These boundary layers, i.e. boundary layer of velocity and the boundary layer of temperature develop as the air flows from the upstream side end of the fins toward the downstream side end, and the velocity and temperature boundary layers on adjacent two fins merge in each other at a point slightly downstream from the upstream ends of these fins to largely decrease the heat transfer rate.
Therefore, in the cross-fin tube type heat exchanger having planar or tubular fins, the rate of heat transfer is inevitably lowered due to the presence of the velocity and temperature boundary layers.
It is, therefore, an effective measure for improving the rate of heat transfer, to prevent the boundary layers from developing or growing.
From this point of view, there have been proposed various improvements in the construction of fin surfaces.
For instance, the specifications of U.S. Pat. Nos. 3,380,518, 3,397,741 and 3,438,433 propose a heat exchanger having a multiplicity of planar fins in which a large number of tabular louver elements are formed in the direction perpendicular to the direction of flow of the air flowing through the gaps between adjacent fins. As the air flows into these gaps, the velocity and temperature boundary layers are formed along the fin surfaces. However, the development of these boundary layers are suppressed at the rear end of each louver element, and are thinned considerably by the action of main flow of air before they reach the next louver element. As a result of such a pattern of air flow, a considerably large effect of cutting the air flow by the leading edges of the louvers, which effect is usually referred to as "leading edge effect of louver", is brought about to improve the efficiency of heat transfer over conventional tabular fins having smooth surfaces. In this heat exchanger, however, each louver element itself has a tabular or flat form.
Japanese Utility Model Application Laid-open No. 17867/1978 discloses a heat exchanger in which each fin is constituted by a corrugated fin plate with ridges extending perpendicular to the direction of flow of the air, and a large number of louver elements which are raised in parallel with one another from the corrugated fin plate.
This type of heat exchanger aims at improving the heat transfer efficiency by a combination effect of promoting turbulency by the corrugation of the fin plate and the aforementioned cutting of air flow by the leading edges of the louvers. In this heat exchanger, however, the louver elements themselves have tabular or flat shape as in the case of heat exchangers proposed by aforementioned United States Patents.
Meanwhile, the specification of U.S. Pat. No. 3,796,258 discloses a heat exchanger in which, as in the case of the above-mentioned Japanese Utility Model Application, each fin is constituted by a corrugated fin plate.
In this case, however, apertures are formed in each fin, instead of the louver elements.
This heat exchanger also improves the heat transfer efficiency by causing turbulent flow of the air, through destroying the boundary layers formed on the surfaces of fins. In this heat exchanger, however, there are no louvers formed in the fins.
The prior art heretofore described more or less achieves the improvement of heat transfer efficiency of the heat exchanger of cross-fin tube type.
Nevertheless, there still is a demand for further improving the heat transfer efficiency of heat exchangers of the kind described.
Apart from the above, the heat exchangers having tabular fins with louvers have a common disadvantage that the rigidity of the fins, particularly in the longitudinal direction of the louvers, is decreased because of a large number of slits which are formed in the fin plate for the formation of the louver elements. This reduction of rigidity is quite disadvantageous and vital from the viewpoint of thinning of fins and facilitation of the assembling work.
Thus, there also is an increasing demand for increase of rigidity of fins of heat exchangers of the kind described.